The present invention relates generally to controlled current sources, and in particular, to current mirrors.
A current source is an electrical element which provides an output current from a very high impedance. The output current magnitude is substantially independent of both the voltage impressed across the current source and the impedance presented thereto. In so-called "controlled" current sources, the magnitude of the output current, rather than being fixed, is a function of a control signal, for example, or a selected circuit parameter such as a resistor value.
A current mirror is a particular type of controlled current source in which the output current is controlled by an input current applied thereto. Current mirrors find use in numerous applications including operational amplifiers, line circuits and electronic switching. Current mirrors known in the art typically inlcude first and second base-coupled transistors. The input current is extended to the collector of the first base-coupled transistor and the output current is provided from the collector of the second base-coupled transistor. Base current for the two transistors is provided by circuitry which diverts a small amount of current away from the collector of one or the other of the transistors.
Ideally, the input and output currents of a unity-gain current mirror should have identical magnitudes. In practice, however, some input/output current deviation is always encountered. In some known current mirrors this deviation is as small as 2/.beta..sup.2 per unit of input current, .beta. being the common-emitter current gain of the transistors comprising the current mirror. In some applications, however, more precise input/output current matching may be required. Moreover, some applications may require a current mirror having higher output impedance than is typically provided by known arrangements.